Telephone terminals connected to a broadband telecommunication system are usually provided with handsets, connected directly or cordlessly, which can send voice messages to, and receive them from, a user. For this purpose, such handsets are usually provided with integrated microphones for receiving the voice message and with loudspeakers for transmitting such a voice message to the user, which are arranged in a common housing of the handset. For reasons of production technique, this housing is usually composed of an upper shell and a lower shell forming a so-called joining edge, i.e. substantially of two housing halves.
To obtain a particularly good sound spectrum in the transmission of a voice message to the user, a resonance volume is usually provided inside the telephone handset in the area of the loudspeaker, because the acoustic signal of the voice message is amplified by resonance in the corresponding frequencies, in dependency of the volume available and in particular of the latter's spatial dimension, and is, therefore, more clearly perceptible to the user. The limited resonance volume in handsets, due to their construction, also limits the frequency range amplified by the resonance and, therefore, also the acoustically perceptible transmission quality of the system. The lower limit of the signal frequencies which can be transmitted well in this way is, therefore, for the usual dimensions of handsets, approximately 400 Hz.
To cover, however, the total frequency spectrum perceptible to the human hearing, which extends on the side of the deep frequencies down to frequencies of approx. 1 Hz, particularly well on the transmission side, in order to achieve a better hearing and transmission quality, handsets of the usual construction are provided with a number of coupling slots. These coupling slots connect the resonance volume inside the telephone handset with the free field outside the telephone handset. This coupling of the resonance volume with the outer air space makes it possible to enlarge the resonance volume in a simple manner. In this way, it is achieved that the frequency range below 400 Hz can also be reproduced or transmitted in an amplified manner, so that a particularly good sound of the voice message can be generated by the loudspeaker. For a reliable coupling with the free field, the coupling slots are usually arranged on the back of the telephone handset. Depending on the application, the handset back can alternatively even be of a completely open design to enable a maximum coupling of the free field. Depending on the configuration of the coupling slots and the coupling they achieve, an amplification of the frequency range down to 100 Hz or even down to 20 Hz can be achieved.
However, the provision of openings on the back of the telephone handset does not only restrict the generally desirable freedom of design in the optical configuration of the handsets, but also the comfort and the immunity from eavesdropping during the operation of such handsets for broadband telecommunication systems. Especially in view of the immunity from eavesdropping, the openings on the back of the telephone handset and the resultant undesired simple radiation of the sound, i.e. the sound leakage into the free field, can cause considerable restrictions. For example, in installations in call centers, this leads to an increased annoyance caused by noise and to an undesired eavesdropping on the part of third parties during confidential conversations.